1. Field of the Invention
This invention relates to a liquid discharge means for a vapor-liquid contacting tray having spout openings in the floor members of the downcomers.
2. Description of the Prior Art
U.S. Pat. No. 3,410,540 to W. Bruckert discloses a vapor-liquid contacting tray of a type adapted for mounting in a vapor-liquid contacting column having a plurality of such trays vertically spaced apart from one another. The disclosed tray is suitable for use in contacting systems such as wash columns, stripping columns, distillation columns and the like. In this type of tray, perforated fluid contact means are provided as the active portion of the tray surface area, and narrow trough-like downcomers are spaced across the tray to provide separate perforated sections of the fluid contact means therebetween, the downcomers serving to transfer liquid to the next lower tray in the column.
In more specific detail, the Bruckert multiple downcomer tray includes perforated fluid contact means providing an active major portion of the tray surface area to support a two-phase fluid dispersion of vapor in liquid for mass exchange, and at least three narrow, trough-like downcomer means evenly spaced across the tray in parallel relation to each other and extending through the perforated fluid contact means as the sole liquid transfer means and comprising a minor portion of the tray surface area such that the fluid contact means is divided into sections of substantially equal surface area per unit length of adjacent downcomer means. Each downcomer means comprises two spaced longitudinal inlet edges each adjacent to a section of the active portion and extending across the tray and imperforate relatively long and deep longitudinal side walls and imperforate relatively short transverse end walls which define a discrete inactive portion of the tray surface area, provide an inlet end section for receiving a fraction of the two-phase fluid dispersion and permitting vapor to disengage therefrom, and provide an outlet end section for collecting disengaged liquid and discharging such disengaged liquid from the tray. Each downcomer means also comprises a liquid sealable outlet means which may suitably be constituted by a floor member attached to the outlet end section of the downcomer, having a multiplicity of spaced-apart spout openings (apertures) therein so constructed that a sealing effect against vapor penetration may be provided by a stable head loss of liquid passing therethrough such that a pool of disengaged liquid may be maintained in the downcomer means.
The above-described Bruckert multiple downcomer tray has been proven to be highly efficient and is widely used in practice as a result of its numerous desirable operating characteristics. The tray is capable of operating efficiently at high liquid flow rates without the problems of excessive entrainment, downcomer flooding, hydrostatic gradient, or high pressure drop. On the Bruckert tray, froth height (the height of the two-phase fluid dispersion of vapor in liquid on the active area of the tray surface) is controllable over wide liquid and vapor flow rates, thereby permitting high rangeability of the gas-liquid contacting system at reasonable vapor phase pressure drop. Furthermore, the hydraulic behavior of the Bruckert tray is accurately predictable. As a result of these operating characteristics, the Bruckert tray may be employed in a vapor-liquid contacting column having tray spacings, i.e., the vertical distance between adjacent contacting trays in the column, lower than is possible for other conventional trays such as those of the cross-flow or corrugated types. In this manner, the Bruckert trays provide more mass transfer capacity per unit column volume with lower power consumption than contacting columns containing such other conventional trays.
Despite the foregoing advantages, however, it has been observed that Bruckert multiple downcomer trays in operation are subject to the disadvantageous phenomenon of penetrative weeping. Penetrative weeping is caused by the impact of the liquid flowing from the downcomers of one tray in a contacting column to the active portion of tray surface area of an underlying tray. The momentum gained by the liquid in its fall adds to the hydrostatic head of the froth on the active portion of tray surface area of the underlying tray, thus producing a localized area of weeping. Weeping involves liquid flow through the gas flow openings of the active portion of tray surface area, in place of the vapor which would otherwise flow through such gas flow openings. Since weeping is synonymous with reduced vapor flow, the vapor that is prevented from passing through the penetrative weeping zone must pass through another part of the tray surface area, thereby creating increased superficial vapor velocities in these other zones. In addition, any liquid which weeps through the tray surface area active portion is not available to contribute to the froth height on the tray surface. All of these factors lead to reduced efficiency, higher pressure drop, higher entrainment and reduced flooding limits of operability for the tray.
Accordingly, it is an object of the present invention to provide an improved vapor-liquid contacting tray. It is another object of the invention to provide improved liquid discharge means for a vapor-liquid contacting tray having spout openings in the floor members of the downcomers.
Other objects and advantages of the invention will be apparent from the ensuing disclosure and the appended claims.